1. Technical Field
The present invention relates to means for installing submarine cables or other elongated elements on a sea bed from a cable ship. A technical problem--in connection with subsea cable installations--is to monitor and make a record of the cable touchdown point in deep waters. Such records are required as documentation of a successful and reliable installation.
2. Description of the Prior Art
When installing cables in areas with low sea currents, a vertical plane can normally be placed through the cable axis from the pay out wheel on the ship to the touchdown point on the sea bed. In these cases it is easy to foresee and make a record of the touchdown point of the cable on the sea bed and the track along which the cable is laid and buried.
Depending on the weight and stiffness of the cable, and the depth of the waters, the sea currents can have considerable influence on the form of the curve followed by the cable on its way down to the sea bed. Instead of following a curve in one vertical plane, the curve can be three-dimensional. Difficulties will then arise when it is required to install the cable along a predetermined route on the sea bed.
Installation of cables on the sea bed is often assisted by ROV's (Remote Operated Vehicles) and/or divers, but when the sea currents are strong and the sea is deep such assistance will be very difficult if not impossible.
When the subsea currents are strong, --the best known solution to the problem is to monitor the touchdown point with an ROV operated from a separate surface vessel. Such a separate ROV solution results--however--in reduced laying speed and increased cost, especially with long cables.
In Norwagian Patent No. 176686 there is described means for determining the geographical coordinates of the touchdown of a cable during its installation by means of touchdown tracker gear. This gear--which rides on the cable a safe distance from the touchdown point during installation--is controlled from the cable ship by means of an umbilical cable. However, --experience has shown that in many cases it will not be possible to get close enough to see the touchdown point with this riding tracker gear.